The purpose of this study is to measure the effect of pre- pregnancy maternal airways responsiveness (hereafter termed "baseline maternal airways responsiveness") on the occurrence of preterm labor, using a case-referent approach to the estimation of the relative odds of airways responsiveness. Specifically, the methacholine challenge test, administered at 6-10 weeks post- delivery, will be used to estimate baseline maternal airways responsiveness and to determine whether baseline responsiveness effects the occurrence of preterm labor independently of maternal cigarette smoking, race, socioeconomic status, age and prior reproductive history. Other intermediate endpoints such as use of tocolytic therapy and the success of such therapy, will be assessed in relation to baseline airways responsivenss. If successful, the research will provide insights into the interrelationship of bronchial and uterine smooth muscle contractility and thereby provide the basis either for further biochemical research into the pathophysiology of smooth muscle reactivity or justification for consideration of clinical trails for prevention of premature labor.